Water-insoluble, water-swellable, hydrogel-forming absorbent polymers are capable of absorbing large quantities of liquids such as water, body fluids (e.g., urine, blood, menstrual fluid), industrial fluids and household fluids and are further capable of retaining such absorbed liquids under moderate pressures. The absorption characteristics of such polymer materials make them especially useful for incorporation into absorbent articles such as disposable diapers, adult incontinence pads and briefs, and catamenial products such as sanitary napkins, and the like.
The development of highly absorbent members used in such absorbent articles are the subject of substantial commercial interest. A highly desired characteristic for such products is thinness. For example, thinner diapers are less bulky to wear, fit better under clothing, and are less noticeable. They are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.
The ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent cores or structures that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of certain absorbent polymers often referred to as "hydrogels," "superabsorbents" or "hydrocolloid" material has been particularly important. See, for example, U.S. Pat. No. 3,699,103 (Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972, that disclose the use of such absorbent polymers (hereafter "hydrogel-forming absorbent polymers") in absorbent articles. Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent cores that take advantage of the ability of these hydrogel-forming absorbent polymers to absorb large quantities of discharged body fluids, typically when used in combination with a fibrous matrix. See, for example, U.S. Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al), issued Jun. 19, 1990, that disclose dual-layer core structures comprising a fibrous matrix and hydrogel-forming absorbent polymers useful in fashioning thin, compact, nonbulky diapers.
In the meantime, prior absorbent structures have generally comprised relatively low amounts (e.g., less than about 50% by weight) of these hydrogel-forming absorbent polymers. See, for example, U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989 (preferably from about 9 to about 50% hydrogel-forming absorbent polymer in the fibrous matrix). There are several reasons for this. The hydrogel-forming absorbent polymers employed in prior absorbent structures have generally not had an absorption rate that would allow them to quickly absorb body fluids, especially in "gush" situations. This has necessitated the inclusion of fibers, typically wood pulp fibers, to serve as temporary reservoirs to hold the discharged fluids until absorbed by the hydrogel-forming absorbent polymer.
More importantly, many of the known hydrogel-forming absorbent polymers exhibited gel blocking when they are used in absorbent articles in a high concentration. "Gel blocking" occurs when particles of the hydrogel-forming absorbent polymer are wetted and the particles swell so as to inhibit fluid transmission to other regions of the absorbent structure. Wetting of these other regions of the absorbent member therefore takes place via a very slow diffusion process. In practical terms, this means acquisition of fluids by the absorbent structure is much slower than the rate at which fluids are discharged, especially in gush situations. Leakage from the absorbent article can take place well before the particles of hydrogel-forming absorbent polymer in the absorbent member are fully saturated or before the fluid can diffuse or wick past the "blocking" particles into the rest of the absorbent member. Gel blocking can be a particularly acute problem if the particles of hydrogel-forming absorbent polymer do not have adequate gel strength and deform or spread under stress once the particles swell with absorbed fluid. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989.
This gel blocking phenomena has typically necessitated the use of a fibrous matrix in which are dispersed the particles of hydrogel-forming absorbent polymer. This fibrous matrix keeps the particles of hydrogel-forming absorbent polymer separated from one another. This fibrous matrix also provides a capillary structure that allows fluid to reach the hydrogel-forming absorbent polymer located in regions remote from the initial fluid discharge point. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989. However, dispersing the hydrogel-forming absorbent polymer in a fibrous matrix at relatively low concentrations in order to minimize or avoid gel blocking may lower the overall fluid storage capacity of thinner absorbent structures. Using lower concentrations of these hydrogel-forming absorbent polymers limits somewhat the real advantage of these materials, namely their ability to absorb and retain large quantities of body fluids per given volume.
In general, increasing the gel strength of hydrogel-forming absorbent polymers can contribute to decrease gel blocking. Gel strength relates to the tendency of the hydrogel formed from these polymers to deform or "flow" under usage stresses. Gel strength needs to be such that the hydrogel formed does not deform and fill to an unacceptable degree the capillary void spaces in the absorbent structure or article, thereby inhibiting the absorbent capacity of the structure/article, as well as the fluid distribution through the structure/article. High gel strength usually obtained by crosslinking. It is believed that crosslinking increases the resistance to deformation of hydrogel-forming absorbent polymer surfaces. However, crosslinking has a deep impact on the absorbent capacity of a hydrogel-forming absorbent polymer. In general, absorbent capacity or "gel volume" has inverse power-law dependence on the level of crosslinking. That is, high crosslinking level results in high gel strength but low gel volume. Gel volume is a measure of the amount of water or body fluids that a given amount of hydrogel-forming polymer can absorb. It is required that gel volume is sufficiently high in order that the hydrogel-forming polymer can absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article.
Another important factor that has to be considered is the liquid permeability of hydrogel-forming absorbent polymers. It has been discovered that the permeability or flow conductivity of the gel layer formed by swelling in the presence of body fluids is extremely important when these absorbent polymers are used in absorbent cores or members at a high concentration in localized or throughout regions thereof. It should be noted that lack of the liquid permeability or flow conductivity of absorbent polymers may directly impact on the ability of resultant gel layers to acquire and distribute body fluids.
Yet another important factor of hydrogel-forming absorbent polymers is the level of extractable polymer material present therein. See U.S. Pat. No. 4,654,039 (Brandt et al), issued Mar. 31, 1987 (reissued Apr. 19, 1988 as Re. 32,649). Many hydrogel-forming absorbent polymers contain significant levels of extractable polymer material. This extractable polymer material can be leached out from the resultant hydrogel by body fluids (e.g., urine) during the time period such body fluids remain in contact with the hydrogel-forming absorbent polymer. It is believed such extracted polymer material can alter both the chemical characteristics (e.g., osmolarity) and physical characteristics (e.g., viscosity) of the body fluid to such an extent that the fluid is more slowly absorbed and more poorly held by the hydrogel. This polymer contaminated fluid is also more poorly transported through the absorbent member. Such a situation can contribute to undesirable and premature leakage of body fluid from the absorbent article. Thus it is desirable to use hydrogel-forming absorbent polymers with lower levels of extractable polymer material.
A further important factor that has to be considered in order to take full advantage of the high concentration of hydrogel-forming absorbent polymers in thinner absorbent articles is the wet integrity of the region or regions in the absorbent member that comprise these polymers. By "good wet integrity" is meant that the region or regions in the absorbent member having the high concentration of hydrogel-forming absorbent polymer have sufficient integrity in a partially wet, and/or wetted state such that the physical continuity of the hydrogel formed after swelling in the presence of body fluids is not substantially disrupted or altered, even when subjected to normal use conditions. During normal use, absorbent cores in absorbent articles are typically subjected to tensional and torsional forces of varying intensity and direction. These tensional and torsional forces include bounching in the crotch area, stretching and twisting forces as the person wearing the absorbent article walks, squats, bends, and the like. If wet integrity is inadequate, these tensional and torsional forces may cause a substantial alternation and/or disruption in the physical continuity of the hydrogel. Such alternation could minimize or completely negate any advantageous fluid distribution (permeability/flow conductivity) property of the hydrogel-forming absorbent polymer. Such alternation and/or disruption may also cause the gel to mobilize and bring about the disclosure of the gel to the surface of absorbent article, therefore cause the so called "gel-on-skin" problem.
Still another important factor of hydrogel-forming absorbent polymers used in thinner absorbent article is the jelly/mushy feel when touching and handling the absorbent article after usage. When hydrogel-forming absorbent polymer is dispersed in region or regions at a high concentration, the swollen gel formed by absorbing body fluids is a gel layer that the particulate is mobile and the gel layer is collapsed when subjected to forces such as pushing, squeezing, etc. when handling the absorbent article after usage. This is why absorbent articles having high concentration of hydrogel-forming absorbent polymer give users or consumers "wet/mushy" feel when touching or handling them from outside.
Therefore, one object of the present invention is to prevent gel blocking phenomena caused in absorbent articles while maintaining required absorbent capacity.
Yet another object of the present invention is to prevent leakage of swollen absorbent materials from absorbent articles.
Still another object of the present invention is to improve the jelly/mushy feel of absorbent articles after usage.
Yet another object of the present invention is to provide thinner absorbent articles.
Still another object of the present invention is to provide thinner absorbent disposable articles such as diapers, sanitary napkins, tampons, and the like.